A capacitor is a passive electronic component that is used to store energy in the form of an electrostatic field, and comprises a pair of electrodes separated by a dielectric layer. When a potential difference exists between the two electrodes, an electric field is present in the dielectric layer. An ideal capacitor is characterized by a single constant value of capacitance, which is a ratio of the electric charge on each electrode to the potential difference between them. For high voltage applications, much larger capacitors have to be used.
One important characteristic of a dielectric material is its breakdown field. The breakdown field corresponds to the value of electric field strength at which the material suffers a catastrophic failure and conducts electricity between the electrodes. For most capacitor geometries, the electric field in the dielectric can be approximated by the voltage between the two electrodes divided by the spacing between the electrodes, which is usually the thickness of the dielectric layer. Since the thickness is usually constant it is more common to refer to a breakdown voltage, rather than a breakdown field. There are a number of factors that can dramatically reduce the breakdown voltage. In particular, the geometry of the conductive electrodes is important factor affecting breakdown voltage for capacitor applications. In particular, sharp edges or points hugely increase the electric field strength locally and can lead to a local breakdown. Once a local breakdown starts at any point, the breakdown will quickly “trace” through the dielectric layer until it reaches the opposite electrode and causes a short circuit.
Breakdown of the dielectric layer usually occurs as follows. Intensity of an electric field becomes high enough to “pull” electrons from atoms of the dielectric material and makes them conduct an electric current from one electrode to another. Presence of impurities in the dielectric or imperfections of the crystal structure can result in an avalanche breakdown as observed in semiconductor devices.
Another important characteristic of a dielectric material is its dielectric permittivity. Different types of dielectric materials are used for capacitors and include ceramics, polymer film, paper, and electrolytic capacitors of different kinds. The most widely used polymer film materials are polypropylene and polyester. Increasing dielectric permittivity allows for increasing volumetric energy density, which makes it an important technical task. The article “Synthesis and spectroscopic characterization of an alkoxysilane dye containing C. I. Disperse Red 1” (Yuanjing Cui, Minquan Wang, Lujian Chen, Guodong Qian, Dyes and Pigments, 62 (2004) pp. 43-47) describe the synthesis of an alkoxysilane dye (ICTES-DR1) which was copolymerized by sol-gel processing to yield organic-inorganic hybrid materials for use as second-order nonlinear optical (NLO) effect. C. I. Disperse Red 1 (DR1) was attached to Si atoms by a carbamate linkage to provide the functionalized silane via the nucleophilic addition reaction of 3-isocyanatopropyl triethoxysilane (ICTES) with DR1 using triethylamine as catalyst. The authors found that triethylamine and dibutyltin dilaurate were almost equally effective as catalysts. The physical properties and structure of ICTES-DR1 were characterized using elemental analysis, mass spectra, 1H-NMR, FTIR, UV-visible spectra and differential scanning calorimetry (DSC). ICTES-DR1 displays excellent solubility in common organic solvents.
Second-order nonlinear optical (NLO) effects of organic molecules have been extensively investigated for their advantages over inorganic crystals. Properties studied, for example, include their large optical non-linearity, ultra fast response speed, high damage thresholds and low absorption loss, etc. Particularly, organic thin films with excellent optical properties have tremendous potential in integrated optics such as optical switching, data manipulation and information processing. Among organic NLO molecules, azo-dye chromophores have been a special interest to many investigators because of their relatively large molecular hyper-polarizability (b) due to delocalization of the p-electronic clouds. They were most frequently either incorporated as a guest in the polymeric matrix (guest-host polymers) or grafted into the polymeric matrix (functionalized polymers) over the past decade.
Chromophoric orientation is obtained by applying a static electric field or by optical poling. Whatever the poling process, poled-order decay is an irreversible process which tends to annihilate the NLO response of the materials and this process is accelerated at higher temperature. For device applications, the most probable candidate must exhibit inherent properties that include: (i) high thermal stability to withstand heating during poling; (ii) high glass transition temperature (Tg) to lock the chromophores in their acentric order after poling.
Most of the polymers, however, have either low Tg or poor thermal stability which makes them unsuitable for direct use. To overcome these problems, one attractive approach is incorporating the nonlinear optical active chromophore into a polymerizable silane by covalent bond to yield an alkoxysilane dye which can be copolymerized via sol-gel processing to form organic-inorganic hybrid materials. The hydrolysis and condensation of functionalized silicon alkoxydes can yield a rigid amorphous three-dimensional network which leads to slower relaxation of NLO chromophores. Therefore, sol-gel hybrid nonlinear optical materials have received significant attention and exhibited the desired properties. In this strategy, the design and synthesis of new network-forming alkoxysilane dye are of paramount importance and detailed investigation of them will offer great promise in the fabrication of new materials for second-order nonlinear optics that will eventually meet the basic requirements in building photonic devices.
In the article “Design and Characterization of Molecular Nonlinear Optical Switches” (Frederic Castet et al., ACCOUNTS OF CHEMICAL RESEARCH, pp. 2656-2665, (2013), Vol. 46, No. 11), Castet et al. illustrate the similarities of the experimental and theoretical tools to design and characterize highly efficient NLO switches but also the difficulties in comparing them. After providing a critical overview of the different theoretical approaches used for evaluating the first hyperpolarizabilities, Castet et al. reported two case studies in which theoretical simulations have provided guidelines to design NLO switches with improved efficiencies. The first example presents the joint theoretical/experimental characterization of a new family of multi-addressable NLO switches based on benzazolo-oxazolidine derivatives. The second focuses on the photoinduced commutation in merocyanine-spiropyran systems, where the significant NLO contrast could be exploited for metal cation identification in a new generation of multiusage sensing devices. Finally, Castet et al. illustrated the impact of environment on the NLO switching properties, with examples based on the keto-enol equilibrium in anil derivatives. Through these representative examples, Castet et al. demonstrated that the rational design of molecular NLO switches, which combines experimental and theoretical approaches, has reached maturity. Future challenges consist in extending the investigated objects to supramolecular architectures involving several NLO-responsive units, in order to exploit their cooperative effects for enhancing the NLO responses and contrasts.
Two copolymers of 3-alkylthiophene (alkyl=hexyl, octyl) and a thiophene functionalized with disperse red 19 (TDR19) as chromophore side chain were synthesized by oxidative polymerization by Marilú Chávez-Castillo et al. (“Third-Order Nonlinear Optical Behavior of Novel Polythiophene Derivatives Functionalized with Disperse Red 19 Chromophore”, Hindawi Publishing Corporation International Journal of Polymer Science, Volume 2015, Article ID 219361, 10 pages, which may be downloaded from the internet at the following URL:http://dx.doi.org/10.1155/2015/219361). The synthetic procedure was easy to perform, cost-effective, and highly versatile. The molecular structure, molecular weight distribution, film morphology, and optical and thermal properties of these polythiophene derivatives were determined by NMR, FT-IR, UV-Vis GPC, DSC-TGA, and AFM. The third-order nonlinear optical response of these materials was performed with nanosecond and femtosecond laser pulses by using the third-harmonic generation (THG) and Z-scan techniques at infrared wavelengths of 1300 and 800nm, respectively. From these experiments it was observed that although the TRD19 incorporation into the side chain of the copolymers was lower than 5%, it was sufficient to increase their nonlinear response in solid state. For instance, the third-order nonlinear electric susceptibility of solid thin films made of these copolymers exhibited an increment of nearly 60% when TDR19 incorporation increased from 3% to 5%. In solution, the copolymers exhibited similar two-photon absorption cross sections σ2PA with a maximum value of 8545GM and 233GM (1GM=10−50 cm4 s) per repeated monomeric unit.
As is generally understood by those skilled in the art, electric susceptibility refers to a dimensionless proportionality constant that indicates the degree of polarization of a dielectric material in response to an applied electric field. The greater the electric susceptibility, the greater the ability of a material to polarize in response to the field and thereby reduce the total electric field inside the material (and store energy). It is in this way that the electric susceptibility influences the electric permittivity of the material and thus influences many other phenomena in that medium, from the capacitance of capacitors. Electric susceptibility is defined as the constant of proportionality (which may be a) relating an electric field E to the induced dielectric polarization density P such that:P=ε0χeE
Where
P is the polarization density, ε0 is the permittivity of free space, χe is the electric susceptibility for the material, and E is the electric field.
The standard definition of nonlinear susceptibilities in SI units is via a Taylor expansion of the polarization's reaction to electric field:P=0+ε0χ(1)+ε0χ(2)E2+ε0χ(3)E3+
The first susceptibility term, χ(1) corresponds to the linear susceptibility described above. While this first term is dimensionless, the subsequent nonlinear susceptibilities χ(n) have units of (m/V)n−1 in SI units. The built-in polarizability P0 is zero, except for ferroelectric materials. The nonlinear susceptibilities can be generalized to anisotropic materials (where each susceptibility χ(1) becomes an n+1-rank tensor). The nonlinear susceptibilities are important in nonlinear optics.
The theory of molecular nonlinear optics based on a sum-over-states (SOS) model was reviewed by Mark G. Kuzyk et al., “Theory of Molecular Nonlinear Optics”, Advances in Optics and Photonics 5, 4-82 (2013) doi: 10.1364/AOP .5.000004 (hereinafter Kuzyk), which is incorporated herein by reference. The interaction of radiation with a single wtp-isolated molecule was treated by first-order perturbation theory, and expressions were derived for the linear (αij) polarizability and nonlinear (βijk, γijkl) molecular hyperpolarizabilities in terms of the properties of the molecular states and the electric dipole transition moments for light-induced transitions between them. Scale invariance was used to estimate fundamental limits for these polarizabilities. The crucial role of the spatial symmetry of both the single molecules and their ordering in dense media, and the transition from the single molecule to the dense medium case (susceptibilities χ(1)ij, χ(2)ijk, χ(3)ijkl), is discussed. For example, for βijk, symmetry determines whether a molecule can support second-order nonlinear processes or not. For non-centrosymmetric molecules, examples of the frequency dispersion based on a two-level model (ground state and one excited state) are the simplest possible for βijk and examples of the resulting frequency dispersion were given. The third-order susceptibility is too complicated to yield simple results in terms of symmetry properties. Kuzyk shows that whereas a two-level model suffices for non-centrosymmetric molecules, symmetric molecules require a minimum of three levels in order to describe effects such as two-photon absorption.
The promising class of (polypyridine-ruthenium)-nitrosyl complexes capable of high yield Ru—NO/Ru—ON isomerization has been targeted as a potential molecular device for the achievement of complete NLO switches in the solid state by Joelle Akl, Chelmia Billot et al., “Molecular materials for switchable nonlinear optics in the solid state, based on ruthenium-nitrosyl complexes”, New J. Chem., 2013, 37, 3518-3527, which is incorporated herein by reference. A computational investigation conducted at the PBE0/6-31+G** DFT level for benchmark systems of general formula [R-terpyridine-RuIICl2 (NO)](PF6) (R being a substituent with various donating or withdrawing capabilities) lead to the suggestion that an isomerization could produce a convincing NLO switch (large value of the βON/βOFF ratio) for R substituents of weak donating capabilities. Four new molecules were obtained in order to test the synthetic feasibility of this class of materials with R=4′-p-bromophenyl, 4′-p-methoxyphenyl, 4′-p-diethylaminophenyl, and 4′-p-nitrophenyl. The different cis-(Cl,Cl) and trans-(Cl,Cl) isomers can be separated by HPLC, and identified by NMR and X-ray crystallographic studies.
Single crystals of doped aniline oligomers can be produced via a simple solution-based self-assembly method (see Yue Wang et al., “Morphological and Dimensional Control via Hierarchical Assembly of Doped Oligoaniline Single Crystals”, J. Am. Chem. Soc. 2012, v. 134, pp. 9251-9262, which is incorporated herein by reference). Detailed mechanistic studies reveal that crystals of different morphologies and dimensions can be produced by a “bottom-up” hierarchical assembly where structures such as one-dimensional (1-D) nanofibers can be aggregated into higher order architectures. A large variety of crystalline nanostructures including 1-D nanofibers and nanowires, 2-D nanoribbons and nanosheets, 3-D nanoplates, stacked sheets, nanoflowers, porous networks, hollow spheres, and twisted coils can be obtained by controlling the nucleation of the crystals and the non-covalent interactions between the doped oligomers. These nanoscale crystals exhibit enhanced conductivity compared to their bulk counterparts as well as interesting structure-property relationships such as shape-dependent crystallinity. Further, the morphology and dimension of these structures can be largely rationalized and predicted by monitoring molecule-solvent interactions via absorption studies. Using doped tetraaniline as a model system, the results and strategies presented by Yue Wang et al. provide insight into the general scheme of shape and size control for organic materials.
Hu Kang et al. detail the synthesis and chemical/physical characterization of a series of unconventional twisted π-electron system electro-optic (EO) chromophores (“Ultralarge Hyperpolarizability Twisted π-Electron System Electro-Optic Chromophores: Synthesis, Solid-State and Solution-Phase Structural Characteristics, Electronic Structures, Linear and Nonlinear Optical Properties, and Computational Studies”, J. AM. CHEM. SOC. 2007, vol. 129, pp. 3267-3286), which is incorporated herein by reference. Crystallographic analysis of these chromophores reveals large ring-ring dihedral twist angles (80-89°) and a highly charge-separated zwitterionic structure dominating the ground state. NOE NMR measurements of the twist angle in solution confirm that the solid-state twisting persists essentially unchanged in solution. Optical, IR, and NMR spectroscopic studies in both the solution phase and solid state further substantiate that the solid-state structural characteristics persist in solution. The aggregation of these highly polar zwitterions is investigated using several experimental techniques, including concentration-dependent optical and fluorescence spectroscopy and pulsed field gradient spin-echo (PGSE) NMR spectroscopy in combination with solid-state data. These studies reveal clear evidence of the formation of centrosymmetric aggregates in concentrated solutions and in the solid state and provide quantitative information on the extent of aggregation. Solution-phase DC electric-field-induced second-harmonic generation (EFISH) measurements reveal unprecedented hyperpolarizabilities (nonresonant μβ as high as −488 000×10−48 esu at 1907 nm). Incorporation of these chromophores into guest-host poled polyvinylphenol films provides very large electro-optic coefficients (r33) of ˜330 pm/V at 1310 nm. The aggregation and structure-property effects on the observed linear/nonlinear optical properties were discussed. High-level computations based on state-averaged complete active space self-consistent field (SA-CASSCF) methods provide a new rationale for these exceptional hyperpolarizabilities and demonstrate significant solvation effects on hyperpolarizabilities, in good agreement with experiment. As such, this work suggests new paradigms for molecular hyperpolarizabilities and electro-optics.
Capacitors as energy storage device have well-known advantages versus electrochemical energy storage, e.g. a battery. Compared to batteries, capacitors are able to store energy with very high power density, i.e. charge/recharge rates, have long shelf life with little degradation, and can be charged and discharged (cycled) hundreds of thousands or millions of times. However, capacitors often do not store energy in small volume or weight as in case of a battery, or at low energy storage cost, which makes capacitors impractical for some applications, for example electric vehicles. Accordingly, it may be an advance in energy storage technology to provide capacitors of higher volumetric and mass energy storage density and lower cost.